High vacuum fuse



Dec. 13, 1932. PRINCE 1,891,111

HIGH VACUUM FUSE Original Filed March 23, 1928 Fig. 7. Fig. 8.

52 Inven 1:161 David c. Prince,

m \\l\\\\\\\\\\\\\\\\\\,, by W c 54 55 HIS Abt/OPneld- DAVII I C. PRINCE, OF SCHENECTADY, NEW YORK, ASSIGNOR TO Patented Dec. 13, 1932 UNITED STATES PATENT OFFICE COMPANY, A CORPORATION OFNEW YORK GENERAL ELECTRIC HIGH .vAoUui/r FUSE Application filed March 23, 1928, Serial No. 264,11). Renewed July 14, 1932.

My invention relates to high vacuum fuses for interrupting electric circuits, and more disclosed, and such-fuses are broadly claimed I,

particularly.v to improvements which facilitate the production of the desired maintainable high vacuum and which are of par-- ticular advantage in' the manufacture of such fuses for controlling circuits which carry considerable current.

In a copending joint application of Bertram Wellman and myself, Serial No. 264,113, filed concurrently herewith and a divisional application thereof, Serial No. 379,976, filed July 22, 1929, both assigned to the same assignee as the present app ication, the principles of construction and the mode of operation in alternating and direct current circuits of fuses comprising a fuse link mounged be tween terminals spaced apart and loca ed in a container or vesselexhausted to have an exceedmgly high and maintalnable vacuum are in the aforesaid applications.

One of the main objects of my present invention is to provide a construction for high vacuum fuses wh ch shall be simple and rugged and which shall particularly facilitate the'manufacture of such fuses for circuits of considerable current capacity, although certain features of the construction are advantageous alsoin the manufacture of fuses in the smaller sizes.

My invention will be better understood from the. following description when considered in connection with the accompanying drawing,.and its scope will be pointed out in the appended claims.

In the accompanying drawing Fig. 1 represent-s one embodiment-of my invention, Fig. 2

i represents the fuse of Fig. 1 as applied to pro- Fig. 3,;Fig. 5 represents an arrangement for securing equal divis on of current through several fuses or fuse links in parallel, Fig. 6

- shows a deta l of a'modified form of fuse terminal utilizing the principles of my invention and Figs. 7 and 8 diagrammatically repthe vessel 5.

resent particular applications of the fuse to electric circuits.

In order to secure the desired functions and results above outlined it is important that the vacuum be extremely high. The better the vacuum the better the fuse will operate. It should be better than a small fraction of a micron (thousandth of a millimeter). Satisfactory operation can be secured with a vacuurn of the order of .01 to .001 micron, but as above stated the best vacuum possible is desired and the best vacuum it is economically practicable to obtain should be had.

It is important to deprive the walls of the fuse-vessel of occluded gases and to free the fuse link and all metal parts within the vessel of such gases' The best technique for obtaining a maintainable high vacuum should therefore be used. In accordance with my invention evacuation to the proper degree of vacuum is facilitated.

Referring to Fig. 1 the fuse link 1 is supported between spaced terminals 3 and 4 in The vessel may be made of suitable glass or quartz or of metal with suitable, provisions for'insulating the conducting parts therefrom. The drawing shows, by way of example, a vessel wit-h a central bulb in which the fuse link is located.

In the arrangement illustrated the fuse supporting terminals 3 and 4. are mounted at the end of the reentrant or stem portions 6 and 7 of the vessel. Inaccordance with my invention I make the terminal members in the form of loops. InFig. 1 they are shaped like a hair-pin. The respective ends of the loop 3 are secured to leading-in conductors 8'and 9, and the ends of loop 4 are secured to leading-in wires 10 and 11. The leading-. in wires may be sealed in the stems 6, and 7 of a glass vessel by a pinch seal. Suitable v conductors which will make a vacuum tight seal with glass may be used. One example of such a conductor is disclosed in Letters Patent to Fink No. 1,498,908 dated June 24, 1924. My loop terminal makes such pinch seals easier to use since each leading-in conductor need have but halfthe cross-sectional area that would be necessary for a single con- To protect the seals from damage from electron bombardment, it is preferable to provide suitable shields as shown at 12 and 13. These shields may be of relatively thin nickle, copper or the like. These shields are spaced from the stems dicated at 12' the shield 12 is supported by a small plate welded to the front of the shield and to the end of the leading-in conductor 9. The leading-in conductor 8 extends through a hole in the front of the shield and may be insulated therefrom as for example by a glass tube as indicated at 12".

After the vessel has been exhausted and sealed off, the leading-in conductors 8, 9 and 10, 11 are connected to the respective outside fuse terminals by which thefuse is to be connected in the circuit to be protected. As indicated in Fig. 2 the fuse may be provided with metal caps which have extensions for mounting in fuse clips .or the equivalent. The fuse capsmay be suitably secured to the vessel, as by cement for example \Vhere a glass vessel is sealed OK at one end, as indicated at 2 in Fig.1, for example, the fuse cap may protect the projection from breakage, when the fuse is handled.

One of the main advantages of the loop or hair-pin shaped terminals is that they permit a large current to be passed through these terminals to heat them to a high temperature" to drive off occluded gases during exhaust.

While the terminal caps are ,off and the lead-- ing-in wires 8 and 9 separate, it is apparent that any desired amount of current can be passed through these wires to heat the loop terminal 3. The loop terminal 4 can be similarly heated by a source connected to the leading-in wires 10 and 11. By connecting a source of current to one of the conductors 8 or 9 and also to one of the conductors 10 or 11, a heating current may be passed through the fuse link 1 to drive off from it occluded gases during the exhausting operation. Durin exhaust suitable means will be used to bad e the occluded gas out of the inner walls of the vessel. Thin metal parts, such as theshields 12 and 13, for example, can be quite effectively heated by inducing high frequency currents therein by suitable means examples of which are well known. Such heating by magnetic induction is disclosed, for example. in Letters Patent to Pratt No. 1,374,679 dated April 12,1921.

After the exhaust has been completed the leading-in wires 8 and 9 are connected to one of the fuse caps and the wires 10 and 11 to the other fuse cap: The current that the fuse link carries when connected in circuit is therefore twice as great as that carried by either leg of either terminal loop. lVithout the loop or hair-pin construction it would be impossible to pass any more current through the supporting terminals than could be car- 6 and 7 and may be supported in any suitable manner. As incurrent during exhaust, it would be apt to be destroyed when the fuse interrupted a circuit while in service. If made as large as they should be to act as proper supports and terminals for the electron discharge, they could not be sufliciently heated during exhaust by the largest current the fuse link could carry.

It is apparent therefore that the loop construction has important advantages both during the exhausting operation and during operation of the fuse in service.

While the loop construction greatly facili tates the final exhaust of the vessel, it is very desirable thoroughly to de-gas all heavy parts before assembling them in the tube for the final exhaust. As pointed out in Letters Patent to Langmuir No. 1,558,436, dated October 20, 1925, after metal has been freed from occluded gas, reabsorption of gas will not readily take place in air or other gases. This fact makes it advisable to pre-de-gas ,such parts since the operation can be carried out more advantageously before the parts are in the vessel.

\Vhile vessels of glass can be made for fuses of any voltage it is feasible and generally will be preferable for the larger currentmoderate voltage fuses to use vessels of metal with suitable seals and insulating means for theconductors which lead into the vessel. For very high voltage fuses, for example fuses for the highest present day commercial voltages which are about 220,000 volts, the size of the fuse vessel is determined principally by the creeping or striking voltage which tends to make the fuse flash outside of the known to those skilled in the art so that these features do not present any unsolved probrelatively large metal vessels are to be used. Examples of such seals are shown in Letters Patent toKruhet al. No. 1,564,690, Decemher 8, 1925, and to Housekeeper No. 1,294,466, February 18, 1919.

The fuse link 1 as heretofore indicated should be very thoroughly freed from occluded gases. This is particularly important because of the fact that the fuse link may not only be raised to a high temperature when the fuse is carrying a large current but when the fuse breaks the circuit it is melted and vaporized. Tungsten has been found to be a suitable metal for the fuse link, but other ,lem where large leading-in conductors or I notproduce a sufficient amount volatilizedto reduce the insulating propertoo close fuse metals may be used so long as the fuse link is mechanically satisfactory and does of gas when ties of the high vacuum dielectric.

The design and construction of the vessel and the mountings for the-fuse link should be such that no part of the fuse porting terminals can at any time during normal operation or blowing of the fuse come to or touch the walls of the vessel.

If such contact should occur it may impair determined testing current is passed throughthe vacuum without breaking the envelope because the hot metal will'cause of a glass vessel, for example, to evolve gas. Or the vessel may be cracked by the hot metal or may-be punctured due to high potential gradients in the glass due to the wire point against it. In either event the paired and its dielectric strength lost.

The distance between the fuse terminals 3 and 4 can be made relatively small so that a short fuselink can be used. This reduces the amount of gas and 'metal vapor which may be evolved when the fuse blows and simplifies the problem of preventing contact with the Wall of the vessel. A suflicient idea of fact that with a bulb 5 inches in diameter and a fuse link of the order of 1 /1 inches in length a current of over 400 amperes has been successfully interrupted at 13,200 volts. The fuse vessel could probably be made still smaller for such service but the example given is suflicient'to indicate the quired for lower voltages as well as higher voltages. i

Experience with fuses ofthe high vacuum type described will enable one to determine rom the appearance of a fuse when a preit, whether or not the vacuum is of the high order desired before the fuse is put into service on a line. If desired, however, any well 'known method of testing such a fuse may be employed to determine whether or not the vacuum has been impaired or is satisfactory.

While the principles of my invention enable fuses to be manufactured for the interruption of heavy currents on lines of high voltage and relatively large capacity the fuse may also be built in smaller sizes and for less severe service. I Fuses 'for the primary circuits of potential transformers have presented some difficulties in the past. For example, cases sometimes occur in which the fuse fails to clear the circuit when by some abnormal condition an excessive current is gradually developed although the circuit may be satisfactorily cleared in case of a short circuit on the potential transformer. In Fig. 2 I-have represented a fuse'embodying my invention applied to the protec 131011 of such potential transformers. The

cated or its sup- .the walls a vacuum is imsize of the vessel reseveral fuse wires when the fuse links winding 15"of the The secondary 16 of the transformer is indias connected to as a voltmeter lies with the fuse and the primary Winding 15 a resistance 18 may be used. The line con ductors 19 and 20 indicateany suitable source across which the potential transformer is connected. In this class of service the duty of the fuse is limited by reason of the fact that the resistance 18 limits the amount of current which can flow even with the transformer 15, 16 short circuited.

' In accordance with my invention it is feasile to mount several'fuse links in parallel within the fuse vessel. An example of such a construction is diagrammatically indicated in Fig. 3 in which four fuse links 21, 22, 23

potential transformer. V

and 24 are mounted in parallel between the rent may also be passed through the several fuse links in parallel in the manner described in connection with passing current through the fuse link 1 of Fig. 1 to provide the necessary heating during the exhausting operation. After the fuse has been sealed off the leading-in conductors may be connected to suitable fuse caps or terminals as described in connection with Fig. 1 and illustrated in Fig. 2. In order that the current may be divided substantially equally between -the the fuse is in service the fuse wires or links should beconnected to the terminals 25 and 26 at the proper symmetrical points in order that the resistance drop in the conducting metal through which the current flows may be substantially equalized. If desired additional leading-in conductors similar to conductors 27 and 28, for example, may be connected to the loop or ring terminal members between the other pair of fuses. As indicated in Fig. 3, leading-in conductor 27 is connected to the loop 25 between the fuse links 21 and 22, and the leading-in conductor 28 is connected to the loop between 23 and 24. Additional leading-in conductors may be connected to the loop respectively between the fuselinks 21 and 2 3, and 22 and 24.

It is apparent that the terminal members for plural 1 link fuses may be mounted and connected in a number of different arrangements. Fig. 4- diagrammatically represents one such modified form of arrangement generally the same as that of Fig. 3. In this case ample terminal of Fig.

the terminal loops 31 and 32 are indicated generally as squares. The leading-in conductors 33 and 34 are indicated as joined to a conductor 35 which will lead to the outside fuse terminal. Similar conductors are indicated at the other end of the device. It is to be understood that the conductors 33 and 34 are not thus connected together to the terminal 35 until the exhausting operation has been completed since these conductors must be kept separate in order to enable the desired local heating current to be passed through the sides of the loop.

If desired, means may be readily provided for insuring that while the fuse is in service the currents carried by the leads 27 and 2S and also 29 and 30 of Fig. 3, or the similar leads 33 and 34 of Fig. 4, shall be substantially equal. This may be done by connecting these leading-in conductors to the opposite ends of a reactance device comprising a number of turns wound in the same direction on an iron core. The conductor which connects to the outside terminal of the fuse, for ex- 4, will be connected to the midpoint of such reactance so that the reactance device is between the fuse terminal and the leading-in conductors.

The principle of operation of such reactance devices will be understood from a consideration of Fig. 5. In Fig. 5 the current is intended to be equalized between four conductors 36, 37, 38 and 39 supplied from a source assumed to be connected to terminals 40 and 41. The conductor 40 is connected to the midpoint of a reactance device 42. Current passing through the conductor 40 to or from the midpoint of the reactance 42 tends to traverse the two sides of the coil in equal amounts. When the current in one side tries to predominate over the current in the other side that current which tends to predominate is choked back and simultaneously a transformer action tends to increase the current inthe side where the current has the lower value. The reactance device 42 therefore tends to keep the current carried by the two wires 36 and 37 equal to that carried by the two wires 38 and 39. In order to equalize the current between the wires 36 and 37, and 38 and 39, the conductors 43 and 44 are connected to the midpoints of similar reactance devices 45 and 46. While such reactance devices at only one end of the comhictors 36. 37, 38 and 39 may be adequate to equalize the current sutiiciently, similar reactance devices 47. 4S and 49 may be provided at the other end of these conductors. a

In applying the principle of equalization shown in Fig. 5 to equalize the current in the conductors 33 and 34 for example of Fig. 4, it will be apparent that the conductors 43 and 44 of Fig. 5 correspond to the conductors 33 and 34 of Fig. 4, and that the conductor 35 of Fig. 4 will, when such a reactance device is used, be connected to the midpoint of the reactance as the conductor 40 is connected in Fig. 5.

Instead of providing a plurality of fuse links in the same vessel it is apparent that a plurality of separate fuse devices may be connected in parallel and'that the current through the individual fuses may be equalized by the arrangement shown in Fig. 5. In this figure each of the conductors 36, 37, 38 and 39 will correspond to an individual fuse. The reactance devices insure that the several fuses will divide the current substantially equally both during normal operation and while the fuses are blowing thus securing a simultaneous blowing of all of the fuses and thereby reducing the duty on each individual fuse.

Fig. 6 represents a modified form of fuse terminal embodying generally the same principles and advantages as the loop or hairpin terminal arrangement of Fig. 1. In this figure, one end of the fuse link is indicated at 50 and is represented as welded to the terminal 51 which is secured in any suitable manner to a leading-in conductor 52, a screw threaded engagement being shown for the purpose of illustration. This conductor 52 is properly sealed into the fuse vessel as above described. In order to heat the fuse terminal 51 effectively to drive out the oceluded gases during the exhausting operation, a coil 53 is provided inside the terminal 51, one end of this coil being connected to the fuse terminal 51 and the other end being extended through and brought out at a suitable point at the side of the leading-in conductor 52 as indicated at 54. A heating current may be supplied to the coil 53 to heat the terminal member 51 to therequired temperature by connecting a source of such current to the leading-in conductor 52 and to the conductor 54. The conductor 54 may be suitably insulatedfrom the conductor 52, as for example, by a small glass tube as indicated at 55. It will be apparent that the arrangement of Fig. 6 represents a modifi cation'of the arrangement of Fig. 1 which still utilizes the principle-of providing a loop through which heating currents may be passed to bring relatively large size terminal members up to the requisite temperature during exhausting. It will be understood, however, that the arrangement in Fig. 6 is diagrammatically illustrated and not accurately drawn to scale.

It will be apparent that the principles of my invention may he applied to high vacuum direct current fuses as well as high vacuum alternating current fuses since in each case it ishighly important to de-gas the terminals very thoroughly. Where very large currents are to be broken by direct current fusesjt is desirable to use several fuse links or fuses in parallel as heretofore described, in order to reduce the duty on the fuse and make proper operation more certain.

In certain relations of utility, apparatus is involved which operates at times on alternating current and at other times on direct current. An example of such apparatus is an electric locomotive which is arranged so as to operate during a part of its journey from an alternating current trolley or third rail, and during other times from a direct current supply. To protect apparatus under such circumstances it is desirable to connect two fuses in series, one fuse being of the high vacuum alternating current type and the other of the high vacuum direct current type. Such an arrangement is diagrammatically indicated in Fig. 7 wherein 56 indicates a device to be protected and 57 and 58 are respectively fuses of the direct current and alternating current type. With such an arrangement the circuit will be protected whether the excess current occurs with an alternating current or a direct current, one or the other fuse being sure to interrupt the circuit even though the other does not.

From the foregoing description it is ap parent that my invention is applicable to fuses of many sizes and-to fuses for circuits of widely different voltages. The fuse is so simple and so comparatively inexpensive when considered with relation to oil circuit breakers of a capacity adequate to interrupt lines subject short circuits, that I anticipate that the high vacuum fuse will have a considerable field of application as a substitute iliary protective device for such as oil switches.

Suitable disconnecting switches will be use with switches provided on each side of the high vacuum fuse so that when the fuse has broken the circuit, the terminals ma be killed and a new fuse safely inserted. uch disconnecting switches are well known and may be either directly operated or remotely controlled. Automatic mechanism may be provided for substituting a new fuse when one fuse has blown and such mechanism may, if desired, e provided to insert fuses a predetermined numberof times in succession, say for example three times, and if the overload still persists the mechanism may lock out as is frequently done in the case of automatic reclosing circuit breakers.

With reference to the use of high vacuum fuses as an auxiliary protection for oil switches I have proposed a combination of apparatus which enables a much smaller oil switch to be used and still produce a safe and economical arrangement for insuring that the circuit shall be cleared in case of an overload such as a short circuit. Oil switches for feeder and line protection, for example, are customarily made a great deal larger than is which the series oil switch to excessive overloads, such as for or as an'auxoverload such as a short circuit the switch must be made a great deal larger and consequently a great deal more expensive than its normal service requires. I propose to employ a high vacuum fuse in series with the oil switch and rely upon the fuse to break the circuit in case the oil switch fails to do so. I can thus use an oil switch which is of the proper size to take care of normal operation and rely upon the fuse to take care of abnormal conditions. According to one arrangement I propose to use an oil switch or suitable quick acting air break switch in shunt with the fuse so that the fuse will be short circuited when this switch is closed. When the circuit is to be interrupted the oil switch in series with the fuse will open first. After this oil switch has opened, its mechanism will be arranged to cause the opening of the switch in shunt with the fuse. If the series switch clears the circuit, the fuse will, of course, carry no current. If, however, the oil switch fails to clear the circuit the opening of the second switch will put the fuse in circuit and this will blow and clear the circuit after may be repaired and a new fuse inserted. As thus arranged the switch in shunt to the fuse would have little duty to perform and it also could be relatively inexpensive. If desired, the switch in shunt to the fuse may be arranged so that it does not open its contacts unless the oil switch fails to clear the circuit. To accomplish this mode of operation several expedients may be provided. For example, the switch in parallel with the fuse may have its trip coil in se ries relation with the circuit to be protected but have that trip coil short circuited when the series oil switch is closed. \Vhen the series oil switch opens, the short circuit will be removed from the trip coil of the switch which shunts the fuse and, if the current is still flowing, the trip coil will be energized and cause theswitch to open thereby inserting the fuse. In order to give time for the oil switch to clear the circuit, the operation of the trip 0011 for the switch shunting the fuse may be retarded by suitable means, examples of which are well known in the art of circuit breakers and oil switches.

The arrangement of the fuse and two switches above described is diagrammatically shown in'Fig. 8, where the oil switch for normally controlling the circuit is indicated at 59.: :The high vacuum fuse is indicated at 60,

cated at 61.

While I have described particular embodi- 7- ments of my invention and particular circuit arrangements in which the fuse may be used division of the circuit current between said it will be apparent to those skilled in the art fuse links. that changes and modifications may be made In witness whereof, I hereunto set my hand without departing from m invention and I this 14th day of March, 1928. therefore aim in the appen ed claims to cover D VID C- R all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is: 75

1. A hi h vacuum fuse comprising an evacuated vessel, a pair of metal terminals within said vessel, a fusible element connected in series with said terminals, said terminals each being in the form of a loop, two sides of the 80 loop terminating each in a leading-in wire extending through the walls of the vessel.

2. A high vacuum fuse comprising a vessel, spaced terminals therein and a fuse link mounted between said terminals each termi- 85 nal comprising a hairpin shaped loop with the legs of each loop extending through the walls of the vessel and sealed therein with a vacuum tight seal, said terminals being relatively 2 large with respect to the fuse link, said terminals and fuse link and the inner walls of said vessel and all other metal parts within the vessel being thoroughly freed of occluded as and the vacuum being as high as it is practicable to attain and at least as high as 95 one one-hundredths of a micron.

3. A fuse comprising a vessel, a pair of terminals located within the vessel, a fusible element located between said terminals, and a pair of leading-in wires extending through 100 the walls of said vessel to each of said terminals whereby each terminal may, independently of the fusible element, be heated whereby occluded gases may be driven off from said terminals. 105

4. A fuse comprising a vessel, a pair of terminals located within said vessel, a fusible element within said vessel connected in series with said terminals, and a pair of leading-in wires extending through the wall of said vessel andterminating in one of said terminals whereby said terminal may be heated, independently of the other terminal by applying a source of current across said leading-in wires i 115 5. A high vacuum fuse comprisinga vessel, spaced terminals within the vessel, a plurality of fuse links supported between said terminals, connections to said terminals extending through the walls of the vessel permitting 120 current to be conducted to said terminals at a plurality of points, said vessel being exhausted to an extremely high degree of vacuum and all parts within the vessel being thoroughly freed of occluded gases. 125 p 6. The combination of a circuit to be interrupted, fuse means for interrupting said circuit comprising a plurality of fuse links in parallel, each fuse link being located in a high vacuum and means for controlling the 

